1. Field of the Invention
The present invention relates to a suspension of a vehicle such as an automobile, and more particularly to a control of an active suspension in relation to a turning of the vehicle.
2. Description of the Prior Art
As a type of active suspensions of vehicles such as automobiles there is known, as shown in, for example, Japanese Patent Laid-open Publication 61-193907, a hydraulic active suspension comprising an actuator provided to correspond to each vehicle wheel so as to change vehicle height at a corresponding position according to supply or exhaust of a working fluid to or from a working fluid chamber thereof, a working fluid supply and exhaust means for supplying or exhausting the working fluid to or from said working fluid chamber, a transverse acceleration detection means for detecting transverse acceleration of a vehicle body, a means for detecting load imposed on a vehicle wheel at an outside of a turn, and a control means for controlling said working fluid supply and exhaust means in accordance with said transverse acceleration.
In such an active suspension, an attitude change of the vehicle body due to a transverse acceleration during a turn is desirably controlled, as the working fluid supply and exhaust means is controlled according to the transverse acceleration of the vehicle body and vehicle heights at respective vehicle wheels are changed so as to suppress the rolling of the vehicle body.
As is well known in the art, the steering performance of a vehicle, such as an automobile, is determined according to the distribution of the roll rigidity between front and rear vehicle wheels. The distribution of the roll rigidity is generally fixed.
The relation between a load on a vehicle wheel, i.e. a tire load (W), and a cornering power (Cp) shows a saturating characteristic as shown in FIG. 9. Therefore, in a relatively high tire load region, the cornering power does not correspondingly increase when the tire load further increases. Therefore, during a high transverse acceleration turn, the cornering power in a front vehicle wheel at the outside of the turn first saturates so that the relative magnitude of the cornering power in the front vehicle wheel decreases as compared to that in a rear vehicle wheel, whereby the vehicle shifts towards the outside of the turn, with the steering performance shifting toward the understeer.
When the vehicle shifts toward the outside of the turn, the slip angle of the front vehicle wheel increases. Since the relation between the cornering force (Cf) and the slip angle (Sa) has also a saturating characteristic as shown in FIG. 10, the vehicle shifts further toward the outside of the turn, resulting in a further shifting of the steering performance toward the understeer.
Therefore, when the distribution of the roll rigidity is shifted more toward the front vehicle wheels by giving predominance to the stability of the vehicle in a lane change or the like, the steering performance shifts more toward the understeer during a high transverse acceleration turn, deteriorating manoeuvrability of the vehicle. In contrast, when the distribution of the roll rigidity is more shifted toward the rear vehicle wheels by giving predominance to the steering performance during a high transverse acceleration turning, the steering performance during a low transverse acceleration turn including a lane change shifts toward the oversteering, thereby deteriorating stability of the vehicle.
The above-mentioned problem exists in both the vehicles equipped with the active suspensions and the vehicles equipped with the conventional passive suspensions. This problem is not solved by the active suspension described in the above-mentioned publication.